What model numbe is your Dell? You should be able to go to Dell's web site and navigate to your model. There should be instructions there as to how to open the case.
That is the service tag, not the serial#, but good enough. It's an Optiplex GX110 and to open the case depends on which type of case you have since it is available in three form factors, if it is a small form factor case, just lay the case flat and press the button on either side then rotate the top up. If it is a small desktop case, it works the same, if you have the mini-tower, it is pretty much the same only you need to lay the tower on its left side and press the two buttons one on the case's top, one on the bottom.